Either Lion or Dog would make sense. But in the show they proposed Dog because the back of the Sphinx is not sloped like the Lion’s—it’s straight and flat like a dog’s. And seeing as how the Dog is guarding the dead, it makes sense for it to be posed at the head of the Valley of the Dead. But again, they think the Sphinx is thousands of years older than the oldest pyramid.
Yes, IIRC, geologist Robert Shoch opined that the Sphinx was originally constructed somewhere around 10,500 B.C. He cited water (rain) erosion patterns on the sides of the Sphinx's body that, in his opinion, could not possibly have occurred later than that.
See the work of Robert Schoch. He’s the guy that stirred that pot and upset the apple cart. Remember, almost everything that the ancients left behind monument wise have something to do with astronomy.